Generally, the hydraulic drive vehicle such as a wheeled hydraulic excavator comprises a hydraulic pump, and a hydraulic motor for travelling which is driven by oil discharged from the hydraulic pump. The output shaft of this hydraulic motor is connected with the input shaft of the transmission, and the rotation of the hydraulic motor is transmitted to the wheels through the transmission. A drain chamber is provided to the hydraulic motor, and the drain oil from the hydraulic motor returns to a reservoir via the drain chamber.
In such a hydraulic drive vehicle as described above, if the hydraulic components or an oil cooler, for example, get damaged, which causes the temperature of pressure oil to be supplied to the hydraulic motor to become high, the viscosity of the pressure oil lowers and the hydraulic motor may be prevented from its proper operation. In this case, there is a possibility that the hydraulic motor may get damaged. If the hydraulic motor is damaged, the discharged oil from the hydraulic pump flows into the drain chamber, and in some cases, it flows into the transmission. As a result, the transmission is filled with the oil therein, and a great resistance comes to act on the transmission so that the travelling performance of the vehicle deteriorates. Moreover, when transmission oil becomes mixed with the oil from the hydraulic motor, the property of the transmission oil may be deteriorated, and this may exert a negative influence upon the operation of the transmission.